The objective of this study is to determine the ultrastructure of amelogenesis in several mammals using the scanning and transmission electron microscopes in order to enhance our understanding of enamel formation and enamel structure. This study will use a new approach by separating the apical region of the ameloblasts from the nascent enamel, then scanning both surfaces of the interface to reveal a detailed functional relationship between the ameloblasts and the enamel they are synthesizing. The comparative aspect of this study will be to examine amelogenesis in the rat, cat, dog, and monkey in order to find a more suitable animal model, one more comparable to human enamel structure than the rat. The differences in the fine structure of the ameloblasts and the structural organization of the enamel among the mammals examined will be noted. Emphasis will be given to uncovering basic principles of amelogenesis which are pertinent to all enamel systems. Particular emphasis will be given to the cellular control by ameloblasts in crystal orientation of rod and interrod enamel, and on a larger scale, cellular control of ameloblast movement resulting in enamel rod decussation. Evidence will be sought pertaining to the function of the papillary layer and its relationship to reduced ameloblasts. New techniques to facilitate the examination of enamel formation and structure will be developed. The information obtained will be used to evaluate results of related studies pertaining to malformed enamel or altered enamel resulting from the ingestion of specific doses of micro-elements.